Definition of Siloxanes:
The term “siloxanes” generally refers to a class of organosilicon compounds with Si—O—Si linkage. Siloxanes may be cyclic or linear. Cyclic siloxanes may have the general formula (R2SiO)n where n equals 3 or greater and R represents an organic ligomer such as for example H or CH3. Linear siloxanes may have the formula R3—Si—(OSiR2)n—O—SiR3 where n equals 0 or greater and R represents an organic ligomer such as for example H or CH3. Examples of cyclic and linear siloxanes and their designations are listed in the table below:
Cyclic siloxanesLinear siloxanesD3: hexamethylcyclotrisiloxaneL2: hexamethyldisiloxaneD4: octamethylcyclotetrasiloxaneL3: octamethyltrisiloxaneD5: decamethylcyclopentasiloxaneL4: decamethyltetrasiloxane
Although not a siloxane, trimethylsilane, (CH3)3SiH, and trimethylsilanol, (CH3)3SiOH, are included in this listing, as small amounts of these compounds may be present in process streams associated with landfill and digester gas, referred to herein collectively as biogas. For the purpose of this disclosure, the term “siloxane” refers not only to linear and cyclic siloxanes as presented in the table above, but also includes trimthylsilane and trimethylsilanol.
Sources of Siloxanes:
Siloxanes are used in a variety of industries. For example, siloxanes may be used in the synthesis of high molecular weight poly(dimethylsiloxane) polymers, in personal care products as emollients, and in detergents as anti-foaming agents. Additional applications include dry cleaning, where siloxanes are used as a more environmentally friendly solvent than traditional chlorofluorocarbons. Siloxanes may also be produced by the semiconductor industry as a by-product of etching applications. Trimethylsilane may be used by the semiconductor industry, e.g., as an etchant gas. Due to their widespread use, siloxanes inevitably find their way into landfills and sewage treatment plants.
Waste Streams Containing Siloxanes:
Biogas from landfills and anaerobic digesters may be comprised primarily of CH4 and CO2. Additional compounds present in biogas may include low to moderate molecular weight volatile organic compounds, chlorine and fluorine-containing halocarbons, sulfur compounds (including hydrogen sulfide, H2S) and siloxanes. For example, a typical biogas stream may contain in excess of 50 ppm total siloxane. In addition to the siloxanes, the biogas stream mayl typically be saturated with water vapor, and may contain up to 1% non-methane volatile organic compounds (VOCs) plus up to and in excess of 1,000 ppm of sulfur compounds that may include mercaptans, thiols and H2S. The concentration of H2S may be in excess of 5000 ppm. It may be desired in many applications to recover and utilize the energy value of biogas as a biomethane fuel to feed on-site generators, for example, for the purpose of power generation.
Need for Siloxane Removal:
The concentration of siloxanes in biogas may be less than a part per million (ppm) to in excess of 50 ppm total siloxane or in excess of 150 ppm total silicon basis. Although the concentration of siloxane may be thought of as low, the effects of siloxanes on downstream process equipment over time may be devastating. This may be due to the fact that siloxanes may undergo thermal oxidation reactions within engines (used to generate power) and thus, may yield microcrystalline silicon dioxide (SiO2) deposits. These deposits may form on the walls of combustion chambers, spark plugs, cylinders, turbine blades, etc., leading to possible abrasion of interior engine parts/components. If the process stream is left untreated, said damage mayl result in frequent engine rebuilds and/or replacement of damaged components. In extreme cases, complete and costly engine overhauls may be required following 6 weeks or less of operation.
Further, as regulations controlling NOx from point-source emissions become more restrictive, catalytic processes, such selective catalytic reduction, are required to further reduce NOx emissions from engines. These catalysts may be readily fouled by any silicon dioxide powder that may elute through the engine, or by any unburned siloxanes that may elute through the engine and react with the catalyst to form deposits, e.g., silicon dioxide deposits.
As a result of understanding their (siloxane) detrimental effects on down-stream processes, it has been discovered that siloxane levels in biogas fuel should be substantially reduced to very low levels prior to the process stream being delivered to engines. For example, it has been discovered that the siloxane level, on an elemental silicon basis, should, according to one embodiment of the invention, be reduced to less than 0.5 ppm, preferably to less than 0.05 ppm, and more preferably less than 0.005 ppm.
Siloxane Removal Processes:
Various methods have been proposed to remove siloxanes from the biogas streams. Adsorption-based systems are the most common. These systems utilize either a single-pass non-regenerated adsorbent bed or a regenerable temperature swing adsorption (TSA) or pressure swing adsorption (PSA) process, or a hybrid of the two systems. The TSA system appears to be the preferred process for removal of siloxanes.
Single-pass, non-regenerable adsorption systems are less complex than the regenerable systems, utilizing one or more adsorbent beds through which the biogas is passed. When the adsorbent becomes loaded to its capacity with siloxane, the adsorbent is removed from the system and replaced with fresh adsorbent. These single-pass systems typically use a carbon-based adsorbent, which require frequent change-out due to the presence and subsequent adsorption of additional organic matter, such as, for example, volatile organic compounds plus H2O and H2S.
The single-pass, non-regenerable systems have operating costs proportional to the amount of siloxanes in the process stream. For process streams containing high siloxane concentrations, the beds require frequent change-out and replacement, which can be expensive and which restricts the application of these systems to all but the low-siloxane level streams. Further, the spent bed may constitute at best “waste” and in lesser cases “hazardous waste,” meaning costs associated with disposal may be significant.
TSA regenerable systems, designed to capture siloxanes and subsequently release the siloxanes to a waste stream, are commercially available. Typically, the siloxanes are adsorbed at or near ambient temperature or pressure on a variety of different adsorbent media such as molecular sieves, activated alumina, zeolites, silica, activated carbon, and diatomaceous earth. For the TSA systems, after the adsorbent is saturated with siloxanes, the flow is generally reversed through the adsorbent bed while the bed is heated to a target temperature, which is maintained for a specified period of time in order to desorb the siloxanes. The waste stream may be vented to atmosphere or flared (burnt) along with some of the process gas in order to meet environmental regulations. Once the desorption operation is complete, the bed is cooled, such as for example, by passing lower temperature gas, such as air, nitrogen or biogas through the bed. Once cooled, the bed is again ready to adsorb siloxanes.
An example of a TSA system for siloxane removal is disclosed in U.S. Pat. No. 7,306,652, where alumina or alumina plus silica (also referred to as silicon dioxide) are used to adsorb siloxanes. The saturated adsorbent is regenerated by passing hot air, or biogas at nominally up to 250° F. through the bed. The hot regeneration gas is reported to desorb the siloxanes from the alumina media, which are then directed to a flare. The regeneration stream can include a slip stream of product gas or external streams used to heat and purge the bed. Although the applicants disclose the use of both aluminum oxide (alumina) and silica media to remove siloxanes, the applicants do not disclose any method for minimizing/preventing reactions leading to the polymerization of siloxanes. Furthermore, the applicants fail to disclose any information relating to the life-time or change-out schedule of the adsorbent.
Hayward et al. (WO 2009/092983) disclose a TSA process for removing siloxanes from landfill and digester gas process streams. Said process employs two resins in a layered bed filter configuration, namely Dowex Optipore V503 resin and an Amberlite XAD4. The purpose of the Dowex Optipore V503 is to remove D3 and D4 siloxanes, while the purpose of the Amberlite XAD4 is to remove the D5 siloxane. Again, the applicants do not disclose any method for minimizing or preventing reactions leading to the polymerization of siloxanes. The applicants also fail to report any information relating to the life-time or change-out schedule of the adsorbent.
Reactions Involving Siloxanes:
Siloxanes are a reactive species that can polymerize and accumulate on an adsorbent over time. For regenerable TSA processes, this may result in reduced siloxane removal performance and the necessity for adsorbent change-outs, thus significantly increasing the life-cycle cost of a siloxane removal plant. Depending upon the characteristics of the biogas feed stream and the siloxane concentrations, many commercial TSA adsorbent beds require replacement up to every 2 to 3 months. To promote longer adsorbent bed life, it has been discovered that it is necessary to minimize the accumulation of siloxanes, siloxane byproducts, and contaminants on the adsorbent.
Siloxanes are known to undergo both acid and base catalyzed polymerization reactions. In the case of D4 siloxane, while not wishing to be bound by any particular theory, the catalyzed acid-catalyzed polymerization reaction may be expected to proceed as follows:

The above reaction is a ring-opening reaction leading to the hydrolysis product. The hydrolysis product further reacts with the siloxane to yield the polymerized product, which may further polymerize:

In another reaction, siloxanes may react with alkali salts to form siloxide salts. For example, hexamethyldisiloxane (L2-((CH3)3Si)2O) may react with sodium hydroxide according to:((CH3)3SO2O+2NaOH→2(CH3)3SiONa+H2O
The impact of the above reactions (both polymerization and siloxide salt formation) may be to accumulate high molecular weight compounds and solids within the pores of the adsorbent media employed by the TSA process. The result of said accumulation will be a degradation of the process performance, culminating in costly change-out and replacement of the adsorbent media.
Hydrogen sulfide, H2S, may be a contaminant present in biogas that also has the potential to undergo reactions with the surface of adsorption media leading to the formation of elemental sulfur and SO2. While not wishing to be bound by any theory, H2S may undergo oxidation reactions with oxidation sites associated with adsorbent media according to:

From the above reaction scheme, H2S may react with surface oxygen to yield water and SO2. SO2 may be further oxidized to SO3, which when combined with adsorbed water may yield sulfuric acid, H2SO4. As siloxanes may undergo acid catalyzed hydrolysis reactions (as described previously), it is, according to one embodiment of the invention, preferable to minimize or eliminate the above reaction. Otherwise, a small amount of oxidation activity may lead to the formation of sulfuric acid, which may lead to the undesired catalyzed polymerization of siloxanes, thereby degrading the media.
The SO2 may also react with H2S as shown below to yield elemental sulfur, which may accumulate on the adsorbent, reducing the adsorbent potential for siloxane.2H2S(g)+3O2(g)--->2SO2(g)+2H2O(g)16H2S(g)+8SO2(g)--->3S8(s)+16H2O(g)According to one embodiment of the present invention, said reactions are preferably minimized.
Heating the media to desorb siloxanes (such as during the regeneration step of a TSA process) will increase the polymerization reaction rates, further promoting accumulation of adsorbed species in the pores of the adsorbent. According to the one embodiment of the present invention, care should therefore be taken in selecting the adsorbent media so that said reactions do not occur at an appreciable rate, either during the adsorption stage or during thermal regeneration of the adsorbent bed. Further, it has been discovered that both adsorption and desorption (i.e. regeneration) temperatures must remain relatively low to avoid the undesirable polymerization reactions, as reaction rates increase exponentially with temperature.
Desired Adsorbent Media Properties:
Many commercial adsorbents, including but not limited to activated carbon, silicon dioxide (often referred to as silica), aluminum oxide (alumina) and zeolite molecular sieves (zeolites), have surfaces which are either acidic, alkaline, oxidative, or contain residual alkali, or a combination of the aforementioned. In certain cases, such as for example activated carbon, aluminum oxides and zeolites, acid-base pairs may be present. Using carbon as an example, during the activation process, functional groups are resident on the surface of carbon. Examples of functional groups associated with the surface of activated carbon may include hydroxides, carboxylic acids, ethers and carbonyls. Depending on the method of activation, the carbon is typically acidic or basic. Activation procedures associated with coal and coconut-based carbons typically yield basic media. Activation procedures associated with wood-based carbons typically yield acidic media.
In the case of aluminum oxides and silica-aluminates, the surfaces tend to have both acid-base pairs. According to one embodiment of the present invention, it has been discovered that both types of sites have the potential to facilitate reactions involving siloxanes. Many silicas are prepared from sodium silicate and therefore contain residual sodium. According to one embodiment of the the present invention, it has been discovered that the residual sodium has the potential to facilitate reactions leading to the formation of siloxide salt. Further, defects are associated with the structure of silicon dioxide. Said defects are often terminated with basic hydroxyl groups, which have the potential to facilitate reactions leading to the polymerization of siloxanes.
Zeolites are another class of adsorbents often employed in separation processes. Zeolites are comprised of a crystalline silica-alumina structure, with sodium being the typical charge-balancing cation present within the pore structure. According to one embodiment of the the present invention, it has been discovered that the sodium has the potential to facilitate reaction leading to the formation of siloxide salts. Zeolites also have defects in the crystalline structure, which may bring about either acidic or basic sites.
From the above examples, according to one embodiment of the the present invention, it has been discovered that the use of commercial adsorbents with acidic or basic surfaces will have the potential to facilitate unwanted siloxane polymerization and/or siloxide salt formation reactions during the TSA siloxane removal processes.
In addition to minimizing reactions involving siloxanes, according to one embodiment of the the present invention, it has been discovered that care should also be taken when selecting adsorbents such that acid gases do not accumulate on the adsorbent, or that acids are not formed on the adsorbent. Otherwise, acording to one embodiment of the the present invention, it has been discovered that acid-catalyzed polymerization reactions involving siloxanes may occur at an increased rate as the number of acid sites increases over time. As discussed earlier, H2S, a significant contaminant associated with biogas, may undergo oxidation reactions with surface oxygen associated with activated carbon leading to the formation of sulfate, which, when combined with water will yield sulfuric acid. According to one embodiment of the present invention, it has been discovered that sulfuric acid has the potential to readily facilitate polymerization reactions involving siloxanes.